Structure and optical properties of pulsed sputter deposited CrxOy∕Cr∕Cr2O3 solar selective coatings

Abstract

Spectrally selective CrxOy∕Cr∕Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottomlayer)∕Cr∕Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy∕Cr∕Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr2O3 layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the CrxOy∕Cr∕Cr2O3 coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (α∕ε) of 0.895∕0.06 even after heat treatment in air up to 300°C for 2h. At higher temperatures, the solar selectivity decreased significantly (e.g., α∕ε=0.855∕0.24 at 350°C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr3+ vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500°C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325°C) and vacuum (up to 600°C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250h at 250°C with a solar selectivity of 0.898∕0.11.